1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method using an electrophotographic process as used in a color laser printer or the like for forming an image on a printing medium such as a sheet of paper by transferring toner images corresponding to different color types of toner onto the printing medium and melting and fixing the toner images by light emitted from a flash lamp.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus for forming an image on a printing medium such as a sheet of paper by using an electrophotographic process is configured so that the image is formed by the following steps (1) and (2).
(1) After a photoconductor drum is evenly charged, the photoconductor drum is exposed to light in accordance with target image data to thereby form an electrostatic latent image. Then, developing agents are supplied to the photoconductor drum and the electrostatic latent image is developed by different color types of toner to thereby form a toner image on the photoconductor drum. Examples of toner generally used include: color toner such as yellow toner, red toner (magenta toner) and blue toner (cyan toner) used for a color image; and black toner (or monochrome toner) mainly used for a monochrome image.(2) The toner image formed on the photoconductor drum is transferred onto a printing medium. The toner image on the printing medium is melted and fixed by a fixing portion.
A recent image forming apparatus has a tendency to need high-speed color printing and a high-quality image on the printing medium. Particularly the fixing portion in the image forming apparatus has large influence on improvement in high-speed color printing and a high-quality image on the printing medium.
As to the fixing portion, there is generally used a flash type fixing unit for melting and fixing the toner image on the printing medium by light emitted from a flash lamp or a heat roller type fixing unit for pressurizing and heating the toner image on the printing medium by a heat roller to thereby fix the toner image.
In the heat roller type fixing unit, because the toner image with colors superposed on one another on the printing medium such as a sheet of paper is melted while pressurized by the heat roller so as to be fixed on a surface of the printing medium, the sheet of paper after fixing has relatively few wrinkles. For high-speed color printing, however, the printing medium such as a sheet of paper carries heat generated in the heat roller when the toner image is fixed. As a result, the temperature of the heat roller surface varies, so that temperature control can hardly be performed at the time of fixing.
Furthermore, when the printing medium is a continuous sheet of paper, a heat roller disconnection mechanism is required at the time of interruption of printing so that the printing medium in the fixing unit is not affected by the heat of the heat roller when printing is interrupted. Accordingly, the fixing unit is complicated in structure.
On the contrary, in the flash type fixing unit, the toner image with colors superposed on one another on the printing medium such as a sheet of paper is melted by heat of light emitted from the flash lamp so as to be fixed on a surface of the printing medium. Accordingly, while the flash type fixing unit can be prevented from coming into contact with the toner image on the printing medium when the toner image is fixed, printing can be performed at a high speed compared with the heat roller type fixing unit. Accordingly, even in the case where toner images corresponding to different color types of toner are color-by-color fixed, fixing can be completed in a short time compared with the heat roller type fixing unit.
Furthermore, even in the case where the printing medium is a continuous sheet of paper, there is no influence of heat of light emitted from the flash lamp at the time of interruption of printing because the flash lamp is switched off when printing is interrupted. Accordingly, the fixing unit can be simplified in structure.
For this reason, the flash type fixing unit adapted for relatively high-speed printing and simple in structure is used popularly in the related-art image forming apparatus.
The configuration of an image forming apparatus having such a flash type fixing unit has been described in JP-A-6-194969 (lines 2 to 20 of page 1, lines 4 to 20 of page 2, lines 30 to 35 of page 6, and FIG. 1) and JP-A-62-254163 (lines 4 to 11 in lower left column of page 1, line 6 in upper left column of page 3 to line 14 in upper right column of page 3, line 18 in upper left column of page 4 to line 8 in upper right column of page 4, and FIG. 1).
More specifically, JP-A-6-194969 has disclosed the configuration of an image forming apparatus having a temporary fixing unit for temporarily fixing images, which are transferred onto a recording medium by a plurality of image forming stations respectively, by flash light exposure in each of the image forming stations. The temporary fixing operation by the temporary fixing unit increases the adsorbability of developing agents transferred onto the recording medium by the image forming stations.
On the other hand, JP-A-62-254163 has disclosed the configuration of a color image forming apparatus in which a heating unit not contacting a transfer medium transport unit is provided at a previous stage of a transfer unit so that a transfer medium is heated by the heating unit before each transfer step to thereby vaporize and scatter water from a sheet of paper to prevent the sheet of paper from being deformed even in the case where the fixing operation of the flash type fixing unit is performed after the heating step.
In the related-art image forming apparatus (particularly, color image forming apparatus) having a flash type fixing unit, when a toner image on a printing medium is melted and fixed by heat of light emitted from a flash lamp in accordance with each of colors in different color types of toner, toner's absorptivity (or reflectivity) of light emitted from the flash lamp varies according to the color of color toner for forming the toner image. Accordingly, if a color image is fixed while all the conditions for fixing different color types of toner are set to be equal to the condition for fixing black color for forming a monochrome image, there is a problem that lowering of fixing characteristic (i.e., fixing rate) is caused by poor fixing of color toner because color toner cannot efficiently absorb the energy of the light emitted from the flash lamp.
On the other hand, in the related-art image forming apparatus, the toner image portion on the printing medium heats because the toner image is fixed by heat of light emitted from the flash lamp. For this reason, when toner images corresponding to different color types of toner are color-by-color fixed, undulations or wrinkles are apt to be generated in the printing medium because of partial shrinkage of the printing medium or shrinkage of molten toner. As a result, there is a problem that transfer missing or transfer failure is caused by the undulations or wrinkles of the printing medium when a toner image corresponding to one color is fixed and then a toner image corresponding to another color is transferred onto the printing medium.